Antenna coupling and tuning system for communication or broadcast receivers



Feb. 9, 1943.

J. W. SHARP ANTENNA COUPLING AND TUNING SYSTEM FOR COMMUNICATION OR BROADCAST RECEIVERS Filed Sept. 30, 1940 Patented Feb. 9, 1943 UNITED STATES PATENT OFFICE ANTENNA COUPLING AND TUNING SYSTEM FOR COMlVIUNICATION OR BROADCAST RECEIVERS 2 Claims.

The present invention relates to communicaticn or broadcast receivers and more especially to improvements in antenna coupling circuits and adjustable circuits or tuners for radio receivers.

It is, therefore, a general object of the present invention to provide novel antenna tuning and high frequency energy transfer circuits.

In coupling a radio receiver to an antenna circuit having very low capacity, it is difficult to obtain maximum transfer of signal energy. Also, it is difficult with an antenna of this type to obtain a signal for use in the receiver which is iree from interference and locally produced noise. In dealing with a small low capacity antenna of very high impedance, such as a simple insulated short rod, difilculties are encountered in obtaining efficient transfer of energy. Antenna coupling circuits of the prior art are only imperfectly adapted to couple antennae of the type just menticned to the input circuits of a radio receiver and these known coupling circuits have inherent deiects which cannot be avoided in practice. .?hey have limited range or frequency spectrum coverage in use, and can transfer only a small portion of a desired frequency or wave length band. Wide variations in coupling are experienced as the adjustable circuits which they employ are adjusted or tuned to select a desired signal source. From a practical standpoint, these previously known antenna coupling arrangements cannot be effectively used especially where space is a factor as in a broadcast receiver for installation in aircraft, or a vehicle such as an automobile, since these known tuners do not lend themselves to compactness of design, flexibility or case of control. Apparatus embodying circuits available prior to this invention could not be designed for ease and cheapness of manufacture, thereby to conserve materials, reduce the cost of material and eliminate unnecessary and costly manufacturing operations. Practical control equipment of suitable design for ordinary or special purposes could not be used effectively with these prior art coupling circuits. In this connection, as stated above, the present invention aims to provide novel tuning apparatus of general use in a wide variety of tunable equipment, and this novel apparatus is especially suited for use in the field of compact radio apparatus such as aircraft and automobile radio receivers employing high impedance low capacity antennae.

Accordingly, one of the major objects of the present invention is to provide improved signal energy transfer from an antenna to the input circuit of a radio or broadcast receiver.

Another important object of the present invention is to overcome the difficulties mentioned above, and other difficulties which are inherently present in prior art adjustable circuits and controls for the adjustable elements of these circuits.

Still another object of the invention is to provide novel circuits, adjustable for tuning, which will permit the maximum transfer of signalling energy from a carrier receiving device, such as an antenna, to a translating device such as, for example, an amplifier in a radio receiver or other repeating device.

A still further object of the invention is to provide novel tunable circuits which will reduce interference caused by undesired oscillations from sources external to the receiver.

A still further object of the invention is to provide a novel antenna coupling circuit in a superheterodyne radio receiver, permitting use of a lower intermediate frequency while at the same time providing separation between the desired frequency and any interfering frequency.

A still further object of this invention is to provide a novel form of coupling between the amplifying portions of a radio receiver and an antenna of high impedance, which coupling will provide uniform sensitivity throughout a desired tuning range.

The manner of realizing the foregoing objects and obtaining general and special advantages discussed above and, also, other and more specific objects of the invention will become apparent as the following detailed description proceeds with reference to the accompanying drawing of which:

Figure 1 is a diagrammatic showing of the present invention embodied in a radio receiver, certain parts of which are shown in a conventional manner.

Figure 2 is a diagrammatic representation of the antenna tuning and coupling circuit of this invention in one form.

Figure 1 of the drawing discloses the invention as being embodied in a radio receiver designed especially for use in a vehicle. It is to be understood, however, that the invention; is not so limited. Referring to Figure 1, reference character 5 indicates the various sections of the receiver which are more or less conventional. All of the circuits of the receiver are not shown but the circuits necessary for an understanding of this invention are shown externally of the diagrammatic representation 5. One of these externally shown circuits is the antenna 6 together with the associated antenna coupling and radio frequency tuning circuit indicated generally by the reference character 7, which includes tunable inductances 8 and 9 and. an antenna trimmer condenser It). The portion 7 of the receiver, which cooperates in a novel manner with radio frequency amplifier I4, is an important feature of the present invention and will be explained more in detail hereinafter in connection with the description of Figure 2 illustrating an equivalent circuit for the antenna 6.

Reference character IE indicates the oscillator frequency control circuit which is shown connected to the oscillator and mixer section of the set. This oscillator control circuit, which in actual practice forms an internal part of the oscillator and functions in the usual manner, comprises a tunable inductance I'I effectively connected in parallel with an inductance I8 which is inductively coupled to a tickler coil I9.

Tuning is accomplished by aplurality of longitudinally movable cores 23, preferably manufactured from ferrous materials. In the embodiment shown in Figure l, which illustrates the fundamental features of this aspect of the invention, three cores 23 are required, which cooperate with inductively tuned antenna tuning circuit inductances 8 and 9 and the inductance I'I forthe oscillator circuit I6. It will be understood that with other types of receivers the cores 23 will vary in number and location. The cores 23 are adjustably secured as at 26 to a yoke member 27 which forms a part of a movable carriage comprising the above mentioned yoke member and a suitable connecting link such as the member 3| which may be reciprocated to simultaneously advance cores 23 into their respective inductances to tune the associated circuits to any desired frequencies.

The antenna coupling circuit I referred to above will now be described in greater detail with reference to Figures 1 and 2 of the drawing. For the elements shown in the same form in Figure 2 and Figure 1, the same reference characters will be used for both figures. The rod antenna 6 and trimmer condenser I9 of Figure l is shown 1n Figure 2 as being equivalent to a signal source HM and a capacity I06 in series, this combination of circuit elements being shunted by a capacity I07. The rectangle 6' encloses this equivalent circuit. The capacity IEl'I of the equivalent antenna circuit just described includes the capacity of the trimmer condenser II] of Figure 1. The capacities I96 and I01 are effectively in'parallel with each other and with the tunable inductance 8 to form a tunable circuit I08 which contains in series the capacity I09. Tuning of inductance 8 and therefore of the tunablecircuit I08 is preferably accomplished by a movable core in the manner set forth in the description of Figure 1. A second tunable circuit III comprising the capacity II2, the coil 9 and the common capacity I09 is coupled to the circuit I08 by the common capacity I69 which has a relatively large value of capacitance with respect to the other capacities in the circuits I08 and II I. The capacity I I2 is preferably made adjustable so that the resonant frequency of the tunable circuit I I I may be adjusted within close limits when using a coil 9 which, by reason of manufacturing tolerances, departs from previously determined design characteristics,

Because of the high impedance signal source having a low value of capacitance, shown as the rod antenna in Figure 1, high inductance is necessary for coils 8 and 8. And, for effective coverage of a sufiiciently large frequency band,

the coils 8 and 9 are preferably long and have a relatively small diameter. In this way the average coupling from one turn to another is low when the tuning cores are withdrawn. Small wire for the coils 8 and 9 is preferable to obtain sufiicient turns in a given length of coil. This results in a low reactance to resistance ratio, which expression is termed the Q of the coil. This relatively low value of Q would, if occurring in known antenna tuning circuits, result in inferior discrimination or station selectivity. When employng coils of the type just described having a value of Q, which may be lower than that of coils designed for capacitive tuned receivers, in the coupling and tuning circuits of the present invention, excellent selectivity and noise discrimination results. Also, material and assembly costs of the coils will be relatively lower as solid wire may be used for winding the coils and cheaper shielding may be used.

The signal input voltage received from the antenna is transferred by the tuned circuits IE8 and HI and appears substantially undiminished in magnitude across the terminals of the capacity H2. This desired and useful signal voltage may be impressed between the anode and grid of a radio frequency amplifier, for example, amplifier I l of Figure l, or it may be utilized in any manner known to the art. In practicing this invention, it has been found that a standard radio receiver equipped with the antenna coupling circuit of this invention gave twice as good a signal to noise ratio at 600 kc. as the same set with a high Q coil and a known coupling arrangement.

In the embodiment disclosed by Figure 1 of the drawing, the antenna coupling arrangement of the invention transfers signal energy to the previously metioned radio frequency amplifier I i which may be of conventional design. The output circuit of the amplifier I4 is untuned, the principal circuit element thereof being the resistor IIA'i which cooperates in the usual manner with the blocking and coupling condenser H6 feeding the mixer grid. The resistor I it together with the radio frequency choke III, the inductance H8 and capacity II 9 provides means to pass a band of frequencies of desired width so that the radio apparatus may be tuned over this band with uniform response if desired, or with greater sensitivity for predetermined portions of the band. The condenser H9 is shown as being adjustable, but is set once to obtain maximum attenuation of signals having frequencies in the neighborhood of the intermediate frequency.

In the following description of the operation of the receiver of Figure 1, which embodies the novel antenna coupling circuit of Figure 2, the theory of signal energy transfer and discrimination against unwanted signals by means of coupled circuits as it applies to this aspect of the invention will be briefly reviewed. Discrimination in any antenna tuning circuit must be sufiicient, for obvious reasons, to prevent strong undesired signals from being passed along to the amplifier or amplifiers and this is especially important in a super-heterodyne receiver where undesired signals reaching the converter grid can produce many spurious responses since the converter acts as a mixer. Cascade tuned circuits or R. F. tuned circuits will provide the necessary discrimination. However, efficient coupling must be provided between the cascaded circuits. The circuits described fulfill this condition of maximum transfer of signal energy, this transfer being efi'iciently obtained as circuits I03 and III can be resonated to the desired signal frequency, and the circuits are readily adjustable or may be adjusted initially to provide optimum coupling.

The common capacitative coupling provided by capacity I09 discriminates particularly against undesired high frequencies since the reactance decreases as the frequency increases and, therefore, the voltage appearing across the terminals of the condenser H2 decreases as the frequency increases. An additional advantage of the antenna coupling circuit provided by this invention also flows from this. Most disturbances, such as ignition noises from an internal combustion engine, are of high frequency, and two sections of a pi filter are provided by the circuit 1 of the invention to attenuate these disturbances which heretofore have been very troublesome. The first of these filter sections includes the inductance 8, the inherent capacity of the antenna 6 plus capacity It] and the capacity N39. The second filter section comprises the capacity I09, the inductance 9 and the capacity H2. Because of the greatly improved selectivity, and in particular because of high attenuation of signals in the image range, it is possible to simplify and reduce the cost of circuit elements which follow, at the same time maintaining their efiiciency. The antenna system described above permits commercially satisfactory image ratios to be secured when an untuned radio frequency stage, such as is provided at H4 and its associated parts in Figure 1 described above, are employed.

While the invention has been described and explained in detail, it is to be understood that the invention may be embodied in other forms, and, therefore, the invention is not limited except as indicated by the terms and scope of the appended claims.

What is claimed is:

1. In a radio receiver, a vacuum tube translating device forming part of said receiver, an antenna of the capacity type and a coupling circuit connecting said antenna to said translating device, said means comprising a first parallel tuned circuit including a capacitive reactance in parallel with an inductive reactance, said antenna and ground being connected across said capacitive reactance, a second parallel tuned circuit including a second capacitive reactance in parallel with a second inductive reactance, said translation device being connected across said second capacitive reactance, said first and second tuned circuits being capacitively coupled by a third capacitive reactance common to and in series in both of said parallel tuned circuits and having a value of capacity relatively large with respect to said first and second capacitive reactances, said inductive reactances being of the movable iron core type, and means for simultaneously moving the cores of said inductive reactances to vary the frequency response of said coupling circuit.

2. In a radio receiver, a coupling circuit for connecting an antenna of the capacitive type to a vacuum tube having a grid and cathode, said circuit comprising, a first parallel tuned circuit including the capacitive reactance of said antenna connected effectively in parallel with a first inductive reactance, said antenna supplying signal voltages to said parallel tuned circuit, a second parallel tuned circuit including a second capacitive reactance connected in parallel with a second inductive reactance, means for connecting said grid and cathode across said second parallel tuned circuit, a third capacitive reactance common to and in series with said first and second parallel tuned circuits for coupling said circuits together, said third capacitive reactance having a value of capacity relatively large with respect to said first and second capacitive reactances, said inductive reactances being of the movable iron core type, and. means for simultaneously moving the cores of said inductive reactances to vary the frequency response of said coupling circuit.

JAMES W. SHARP. 

